As communications devices such as cell phones, PDAs, mobile televisions, personal navigation devices, personal media players and a myriad others continue to become more commonplace in the modern society, the performance requirements of communications systems are increasing at a staggering pace. Today, these devices are expected to perform with increasing reliability and improved capabilities while maintaining a competitive price point. The receiver is a determining component in a communications device's performance and cost. The receiver's function is to receive an often significantly distorted and attenuated signal and convert it into a signal that can be used by the other components in the system. The quality of the signals produced by the receiver is a limiting factor in the performance of communications systems and manufacturers continuously strive to improve this aspect of receiver design.
Generally, a communications system includes a transmitter communicating with a receiver over a communications channel. The transmitter sends a signal over the communications channel to a receiver located in a device. The communications channel can be cable wire, air, or other medium. The receiver can receive the signal from an antenna or through direct wire transmission. Generally, before information contained in the signal is used in the device, the signal's power is increased through amplification. Hence, the portion of the receiver where amplification takes place is typically referred to as the RF front end. Usually, the RF front end is a circuit incorporating one or more low noise amplifiers (LNAs). The gain in the RF front end can be controlled by RF automatic gain control (AGC) circuits.
Further, the power of signals received at a receiver can vary significantly. Namely, due to attenuation, a signal's power declines as the signal travels away from the transmitter. For example, a signal sent to a receiver through the air may have significantly lower power further from the transmitter than close to it. The difference in power can be in the order of several magnitudes. Hence, a device in a traveling vehicle, for instance, may observe severe fluctuation in incoming signal power as it travels from the proximity of one radio tower into the proximity of another radio tower. As a result, the receiver must be able to amplify incoming signals in a broad range of powers to produce signals with desired power and other desirable characteristics.
Existing devices apply a single amplification routine to all incoming signals. However, this results in a non-optimal routine being applied to signals in much of the power range. Especially in the case of strong signals and blockers (undesired received signals), a RF front end designed for low power signals and optimized for low noise figure and input matching will produce intolerable nonlinearity and distortion. What is needed is a mechanism for amplifying incoming signals in a broad range of powers that produces signals with desired power, linearity, noise figure, and other desirable characteristics across the range.